1. Field of the Invention
Embodiments of the present invention relate to endoscopes including a light guide, and specifically relate to endoscopes including a light guide made of lead-free glass.
2. Description of the Related Art
Medical endoscopes need to illuminate an object to observe an inside of a dark body cavity. Thus, a light guide (LG) is used for guiding light generated by a light source apparatus to an illumination section disposed in a distal end portion of an insertion portion of the endoscope.
A light guide has a configuration in which numerous optical fibers are bundled. As illustrated in FIG. 1, an individual fiber 10 includes a core 11 that transmits light, and a clad 12 provided on an outer circumferential portion of the core 11, the clad 12 reflecting light so as to prevent the light from leaking from a side face of the core to the outside. A high-refractive index glass is used for the core 11 and a glass having a refractive index lower than that of the core 11 is used for the clad 12.
For core glass for normal LGs, glass containing lead, i.e., what is called lead glass, is used. This is because lead glass has the advantages of, e.g., low price, easy fiber manufacturing and good transmittance. In recent years, in order to respond to environmental problems, glass with no lead used therein (hereinafter also referred to as “lead-free glass”) has been developed. For example, Japanese Patent Application Laid-Open Publication Nos. 2004-256389 and 2004-277281 each disclose lead-free aluminosilicate glass.
Here, with medical endoscopes, not only normal-light imaging using white color light, but also various types of special light imaging using wavelength characteristics of illuminating light are performed. For example, narrow band imaging (NBI) is a method in which hemoglobin in blood is irradiated with light having two narrowed wavelength bands that are easily absorbed in blood, for example, illuminating light in a first wavelength band of 390 to 445 nm and light in a second wavelength band of 530 to 550 nm to highlight capillary vessels in a superficial layer of a mucous membrane and a fine pattern of the mucous membrane, thereby easily discriminating a tumor tissue. In the method, image information in a blue color light range is very important.
Also, in some of the models of the medical endoscopes, for example, endoscopes for small intestine, large intestine, a pancreatobiliary region or renal pelvis and ureter, an X-ray may be applied to confirm a position of the endoscope inserted in a body.
Also, Japanese Patent Application Laid-Open Publication No. 2009-7194 discloses lead-free glass containing a rare-earth oxide and having radiation blocking ability. Furthermore, for communication fibers used under strong radiation exposure environments such as nuclear power plants, quartz-based optical fibers that contain a CH group or an OH group for enhancement of the radiation resistant characteristic are used. Furthermore, Japanese Patent Application Laid-Open Publication No. 59-128501 discloses an outer circumferential portion of a broadband optical fiber containing quartz as its main material, which is used under a high radiation environment containing neutrons, being covered by metal coating.
Furthermore, Japanese Patent Application Laid-Open Publication No. 2011-116621 discloses glass for a light guide for an endoscope.